Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability with any implantable medical device or in any implantable medical device system.
As shown in FIG. 1, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 102 formed of titanium for example. The case 102 typically holds the circuitry and battery 104 necessary for the IPG to function. The IPG 100 is coupled to electrodes 116 via one or more electrode leads (two such leads 114a and 114b are shown), such that the electrodes 116 form an electrode array 110. The electrodes 116 are carried on a flexible body 118, which also houses the individual signal wires 112a-112p coupled to each electrode. The signal wires 112a-112p are connected to the IPG 100 at one or more lead connectors 106 fixed in a header 108, which can comprise an epoxy for example. In the illustrated embodiment, there are eight electrodes on lead 114a, labeled E1-E8, and eight electrodes on lead 114b, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. In a SCS application, electrode leads 114a and 114b are typically implanted on the right and left side of the dura within the patient's spinal cord. These proximal end of leads 114a and 114b are then tunneled through the patient's flesh to a distant location, such as the buttocks, where the IPG case 102 is implanted, at which point they are coupled to the lead connector(s) 106.
As shown in the cross section of FIG. 3, IPG 100 typically comprises an electronic substrate assembly 120 including a printed circuit board (PCB) 122, to which various electronic components 124 are mounted, some of which are described later with reference to FIG. 4. The IPG 100 can further include a telemetry antenna or coil (not shown) for communicating with an external device such as a patient or clinician controller (not shown), which telemetry antenna can be positioned in the header 108 or in the case 102. A charging coil 130 within the case 102 receives a magnetic field 150 from an external charger 200 to charge the IPG's battery 104, as explained further below.
To avoid having to surgically replace the IPG 100 when its energy is depleted, the IPG 100's battery 104 is typically rechargeable via an external charger 200 as just mentioned. FIG. 2A shows a plan view of the external charger 200, which includes a housing 204 configured to be hand-holdable and portable (akin in size and shape to a cell phone for example), and which is typically constructed of rigid plastic. An indicator 206, such as one or more light-emitting diode LED(s), can display multiple patterns or colors to indicate the status of the external charger 200, such as whether the external charger is actively producing the magnetic field 150, or to indicate the status of a battery 220 in the external charger 200, described further below. An on/off switch 208 activates the external charger 200 to produce the magnetic field 150.
As best shown in the cross section of FIG. 3, the external charger 200 is powered by a battery 220, which can be recharged as described further below. The external charger 200 can contain one or more printed circuit boards 210 and 212 that contains the circuitry 214 needed to implement its functionality, which circuitry is described further with respect to FIG. 4. The hand-holdable external charger housing 204 can include a top surface 204a and a bottom surface 204b, which if formed in separate portions may be snapped together or connected by other means. Note that the top and bottoms surfaces 204a and 204b are substantially planar (within 10 degrees), although they can be slightly angled with respect to one another as shown to accommodate the thickness of the battery 220 if necessary.
The external charger 200 includes a primary charging coil 230, which when provided with an AC current (e.g., when on/off switch 208 is pressed), produces the magnetic field 150 to charge the IPG's battery 104 via inductive coupling. The magnetic field 150 induces an AC current in the IPG's charging coil 130, which current is then rectified and used to recharge battery 104 in the IPG 100. Battery 104 in the IPG 100 may comprise a rechargeable polymer lithium-ion battery for example. As one skilled in the art understands, the efficiency of energy transfer between the coils 230 and 130, i.e., their coupling, is improved if the planes of the coils are parallel; if the axes around which they are wound are collinear; and if the coils are as close as possible. This means of inductively transferring energy from the external charger 200 to the IPG 100 can occur transcutaneously, i.e., through the patient's tissue 160.
As noted, the external charger 200 is powered by a battery 220, such as a rechargeable lithium-ion battery, which may from time to time become depleted. To recharge the external charger's battery 220, the external charger 200 can be placed in a charging cradle 250, as shown in FIG. 2B. The cradle 250 may have a plastic housing 252 with an indentation or recess 254 generally shaped to hold the external charger housing 204. Cradle contacts 256 within the recess 254 are designed to meet with external charger contacts 202 which pass through the bottom surface 204b of the external charger housing 204 (see FIG. 3), such that when the external charger 200 is resting in the cradle 250, the contacts 202 and 256 touch, and which allows power to transfer from the cradle 250 to the external charger 200, as described further below. The cradle housing 252 can contain cutouts 258 to assist a user in placing and removing the external charger 200 from the cradle 250. LED indicator 206 on the external charger 200 can be used to indicate when charging of the external charger's battery 220 is occurring, or when charging of battery 220 is complete. The cradle may also contain one or more LEDs to indicate the status of charging the battery 220, although this is not shown. The cradle 250 itself, as is typical, can be plugged into a wall socket via power cord 260.
In lieu of a cradle 250, one skilled will realize that battery 220 in the external charger 200 can be recharged by other means. For example, circuitry in the cradle 250 can be formed in a power cord terminating in a coaxial plug (akin to contacts 256), which plug can then be inserted into a coaxial port (akin to contacts 202) on the external charger 200.
FIG. 4 shows the circuitry within the cradle 250, the external charger 200, and the IPG 100. The operation of such circuitry is generally well known, and thus only briefly described.
AC power from a wall socket 401 is transmitted from the power cord 260 to a transformer 262 in the cradle 250. The transformed power is rectified 264 to a DC voltage (e.g., Vdc=5V) and presented to the cradle contacts 256. Capacitor 266 assists in smoothing or filtering the produced DC voltage. (One skilled will recognize that the transformer 262 and rectifier 264 can also be positioned in line with the power cord 260, and hence outside of the cradle 250).
When the external charger 200 is placed in the cradle 250, Vdc is presented to external charger contacts 202, where it is met by circuitry in the external charger 200 for charging its battery 220. Such circuitry includes a charging circuit 234, battery protection circuitry 236, and a MOSFET switch 237. A small noise-decoupling capacitor 232, e.g., 0.1 μF, can be placed between the external charger contacts 202 at the input of the charging circuit 234. Charging circuit 234 provides a charging current, Ibat1, to charge the battery 220, which circuit 234 may comprise Part No. LTC1733, manufactured by Linear Technology Corp. Charging circuit 234 may charge the battery 220 in different charging modes, such as: a trickle charging mode which produces a small Ibat1 until the battery voltage, Vbat1, reaches a first threshold; a normal charging mode which charges the battery with a higher constant current Ibat1 thereafter; and a constant voltage charging mode when Vbat1 reaches a second higher threshold, which charges the battery 220 still further until Ibat drops to an insignificant value. To prevent overcharging or unwanted discharging of the battery 220, battery protection circuitry 236, upon sensing such a condition, can disconnect the battery 220 from its ground 239 by opening switch 237. (Ground 239 may different from the system ground used by other components in the external charger 200). Battery protection circuitry 236 may comprise Part No. NCP802, manufactured by ON Semiconductor, for example.
The external charger 200 also comprises circuitry to produce the magnetic field 150 used to charge the IPG's battery 104, including: a regulator 238, such as a low drop-out voltage regulator, for supplying a clean and predictable power supply voltage, Vdd, for the external charger's electronics; control circuitry 240, which may comprise a microcontroller for example; an amplifier 242 for driving an AC current Ic though the charging coil 230; and a tuning capacitor 244 used to set the frequency of the magnetic charging field 150 produced, which may be 80 kHz in one example. The IPG 100's components are well known, and have been previously described: charging coil 130 receives the magnetic field 150; its induced current is rectified 132 to a DC voltage; which is used to charge the IPG's battery 104, perhaps via charging/protection circuitry 134 as shown.